Circuit boards are either hand soldered or automatically soldered with an automatic soldering system, such as the systems manufactured by Zeva Electric Corporation, Electro-vert and the like. Typical automatic soldering machines include wave soldering machines and drag soldering machines. If the circuit board contains a large number of components, the components are usually soldered enmasse, after their placement on the board, in an automatic soldering machine. If heat or solvent sensitive components are to be included on the board, such components are usually hand soldered onto the board after the automatic soldering operation. The conventional automatic soldering machine, such as the Zeva brand soldering machines have a longitudinal frame including a flux coating stage, a drying and preheating stage, and a soldering stage. The boards are attached to a carrier carried by conveyer chains, or by a conveyor chain having integral fingers to carry circuit boards, that run the length of the longitudinal frame past each stage. A board is loaded into the machine at one end, carried to a flux coating stage wherein flux is applied to the solder side (underside) of the board, passed to the drying and preheating stage where the flux coating is dried and the board is preheated, and then immersed and transported across the surface of a hot molten solder bath or molten solder wave to complete the solder operation. The board is then deposited at the exit end of the machine.
Soldering machines of the type described above have seen extensive use. However, there are operational problems associated with these machines. If the boards inadvertently slip from the carrier or warp from the preheating, they frequently may jam in the machine and stop the conveyer chain. This often causes destruction of the board because the chain drives are powerful and can break or warp the jammed boards. In addition, the unsoldered and loose components can separate from the board. The removal of the jammed board from the machine can be a hazardous operation if the board is jammed in the solder bath. In addition, molten solder from a wave soldering machine can be channeled by a jammed or warped board diverting hot molten solder into the machine or surrounding area. The operation of a conventional automatic soldering machine does not always make the most efficient use of an operator. For many machines, a single operator must feed the boards in at one end, proceed to the other end of the machine, which range in length from 10 feet to over 20 feet, to remove the soldered boards. Accordingly, the operator spends valuable time walking from one end of the machine to the other during operation of the machine.
At the present time, the majority of circuit boards are fabricated so that the electronic components are mounted on one side of the board, the component side, and the components are soldered on the other side of the board, the solder side, with the leads extending from the component side of the board through the apertured solder junctions on the board to the solder side of the board.
Recently, surface mount devices have come into commercial use. Surface mount devices, commonly called SMDs, are electronic components that are mounted on one side of the board with an adhesive and soldered to the solder junctions on the same side of the board. When SMDs are soldered with an automatic soldering machine, the side of the board with the SDMs is immersed into the surface of the solder bath or wave. Problems have arisen from soldering SMDs with automatic soldering machines. It is believed that gas pockets form at the juncture between the SMDs and the board during the soldering operation which prevents the flow of solder thereto. To overcome this problem, wave soldering systems have been modified by adding a second solder wave flowing in an opposite direction to the first wave so as to direct solder into the junction between the SMDs and the board in two directions to sweep out gas pockets such as the system manufactured by the Electrovert USA Corporation. In a drag soldering operation, the surface of the solder bath is maintained free of ripples and waves, except for the ripples or eddies created by the board as it is transported across the surface of the solder bath.
In both the drag soldering machines and the wave soldering machines, once the board is fed into the device, there is no simple way to retrieve the board until completion of the soldering operation. The machines can be stopped, i.e. the chain conveyor can be stopped, but the machines cannot be reversed at will to return the board to the feed station. Reversal of the machine is not practical because the board would have to proceed back through and interact with each stage it has passed. If the board had partially slipped from the carrier it would jam in either direction. The machine is stopped to retrieve the board from the chain conveyor. This can be hazardous when the board is over the soldering station or the drying and preheating station where there are exposed heating elements.